Allow inline CSS for debug handlers to make prototyping easier. These
are generally not accessible to the public and the small risk of CSS
injection via user content seems acceptable.
Also allow form submissions on the same domain, instead of banning all
forms. An example of such form is
http://webhooks.corp.ts.net:6359/debug/private-nodes/
Updates #3576
Signed-off-by: Andrew Lytvynov <awly@tailscale.com>
Add a few helper functions in tsweb to add common security headers to handlers. Use those functions for all non-tailscaled-facing endpoints in derper.
Signed-off-by: Andrew Lytvynov <awly@tailscale.com>